A conventional turbo fan engine uses the core engine to drive a bypass fan mounted near the engine intake. Fan blades on the bypass fan drive a core flow into the core engine and a bypass flow around the core engine. The bypass flow combines downstream with the core exhaust flow to provide propulsive thrust.
A casing assembly extends around the outside of the fan to provide an outer wall of a flow annulus through the fan. The fan blades themselves are not normally provided with blade platforms, and so a number of separate circumferential wall inserts or “annulus fillers” are mounted on the outside of the fan rotor disc, in between the fan blades, to form the inner wall of the flow annulus through the fan.
The annulus fillers are typically mounted on the fan rotor disc using a hook arrangement, such as the one described in International Application PCT/GB93/00372 (published as WO93/21425). Here, each annulus filler is provided with a pair of hooks which extend radially inwardly from the filler to engage correspondingly shaped hooks provided on the outer face of the fan rotor disc. The hooks on the filler must be maintained in axial engagement with the hooks on the fan rotor disc, and one or more separate thrust rings is typically provided for this purpose.
A similar configuration is shown in FIG. 1. A blade 2 is connected to a disc 4 at a radially outer face of the disc 4 by an interlocking configuration, such as a dovetail joint. A plurality of blades 2 are assembled onto the disc 4 around the circumference of the disc 4 to form a rotor. As described previously, an annulus filler 6 is provided between adjacent blades 2 so as to form the inner wall of the flow annulus through the fan. The annulus filler 6 is mounted to the disc by a pair of annulus filler hooks 8, 10 which engage with correspondingly shaped disc hooks 12, 14. The hook arrangement provides radial retention of the annulus filler 6 against centrifugal loads experienced during operation of the rotor. A plurality of annulus fillers 6 are provided between each pair of adjacent blades 2. To ensure that the annulus filler hooks 8, 10 are maintained in engagement with the disc hooks 12, 14, the axial position of the annulus filler 6 with respect to the disc 4 is fixed by a nose cone support ring 16. The nose cone support ring 16 covers the full circumference of the rotor and retains each of the annulus fillers 6. The nose cone support ring 16 is connected to an arm 18 of the disc and also to an arm 20 of the annulus filler 6. Consequently, the axial position of the annulus filler 6 is fixed so that the hooks remain engaged. During operation, the nose cone support ring 16 also bears a component of the centrifugal load of the annulus filler 6 which creates hoop stress in the nose cone support ring 16.
The nose cone support ring also functions as the primary fixation point for a nose cone of the turbomachine. The nose cone creates smooth airflow into the fan, particularly at the root of the blades, and also must be capable of withstanding bird strikes and preventing build up of ice. The nose cone 22 is located on an annular shoulder 24 of the nose cone support ring 16 and is connected at positions around the nose cone support ring 16 via abutting radial flanges 26.
The connection between the nose cone support ring 16 and the nose cone 22 is enclosed by a cover portion 28. The forward (upstream) axial end of the annulus filer 6 has a tongue portion which is received under a lip portion 32 of the cover portion 28. A similar arrangement is provided at the opposite axial end for mating with a rotating seal element 34.
A hook-type mounting arrangement such as the one described in International Application PCT/GB93/00372 and as shown in FIG. 1 requires that dedicated, load-bearing attachment features such as hooks must be formed on the outside of a forged fan rotor disc and this adds to the cost and complexity of manufacturing the fan rotor disc.
In addition, safely engaging the hooks with one another may be difficult and time-consuming because, in practice, the hooks tend to be obscured from view by the adjacent blades and by the annulus filler itself during assembly. Failure to safely engage the hooks increases the risk of annulus filler detachment under a centrifugal load during rotation of the fan.
During a bird strike or fan blade off (FBO) event, a fan blade may be deflected and apply a circumferential load to an adjacent annulus filler. Tests have shown that some prior art annulus filler inserts secured using hook style fixings may be vulnerable to detachment under these circumferential loads.
The present invention seeks to provide an improved annulus filler assembly, and in particular seeks to provide an annulus filler assembly which addresses one or more of the specific problems referred to above.